FM-CW radars utilizing millimeter waves have been developed, for example, as in-vehicle radars. FM-CW radars detect targets by transmitting and receiving radio waves obtained by performing frequency modulation (FM) on continuous waves (CW). In other words, FM-CW radars transmit a transmission signal repeating an upstream-modulation section in which a frequency gradually increases and a downstream-modulation section in which the frequency gradually decreases, receive a reception signal including a reflection signal from a target, and calculate the relative distance and the relative speed of the target in accordance with a frequency spectrum of a beat signal, which is a signal indicating the frequency difference between the transmission signal and the reception signal. Since, normally, the relative position and relative speed of a target is not constant, the above-mentioned operations are repeated at a constant frequency in order to acquire the relative position and relative speed of the target every time the operations are performed. Since targets are distributed within a detection azimuth range, directions of the targets within the detection azimuth range can be calculated by performing the above-mentioned operations for a beam facing toward a predetermined direction and by sequentially changing the beam direction.
When a single target exists, a single projecting portion appears in a frequency spectrum of a beat signal based on a reflection wave from a target in each of an upstream-modulation section and a downstream-modulation section. Thus, the peak frequency of the projecting portion of each of the beat signal in the upstream-modulation section (hereinafter, referred to as an “upbeat signal”) and the beat signal in the downstream-modulation section (hereinafter, referred to as a “downbeat signal”) is calculated, and the relative distance and the relative speed of the target are calculated from the two peak frequencies.
However, when a plurality of targets exists in substantially the same direction, a plurality of projecting portions appears in a frequency spectrum of each of an upbeat signal and a downbeat signal of an identical beam. Thus, it is necessary to determine, from among the plurality of projecting portions, which combination of projecting portions is generated due to the existence of an identical target (hereinafter, referred to as “pairing”). However, as the number of detected projecting portions increases, it takes a longer time to perform pairing. In addition, since the number of combinations increases, there is a larger possibility to perform wrong pairing. Thus, there are a problem in that the number of targets that can be detected within a limited period of time is limited, a problem in that providing an arithmetic processing unit capable of performing a high-speed arithmetic operation in order to detect many targets increases cost, and a problem in that it is difficult to acquire accurate relative distance and speed when wrong pairing is performed.
In order to avoid the above-described problems, actual radars increase the accuracy by performing filtering processing in which context is taken into consideration so as not to depend only on a single pairing operation. However, it is important not to perform wrong pairing from the beginning.
Thus, as disclosed in patent document 1, pairing is performed by regarding a combination of projecting portions appearing in a frequency spectrum of a reception signal, intensities of the projecting portions being substantially equal to each other, as being caused by an identical target.
In addition, a technology for setting the gradient of upstream modulation and the gradient of downstream modulation such that a moving distance by the amount corresponding to a Doppler shift frequency corresponds to a moving distance by relative speed at a predicted time in the future is disclosed in patent document 2. With this arrangement, distance can be calculated without performing pairing.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 4-343084
Patent Document 2: Japanese Unexamined Patent Application Publication No. 6-94829
However, in the method described in patent document 1, if a plurality of projecting portions whose reception signal intensities are substantially equal to each other appears, a combination of a pair of projecting portions may not be determined.
In addition, in the method described in patent document 2, although pairing is not necessary for calculating distance, relative speed cannot be calculated.